1. Field of the Invention
The present invention relates to optical communications, and in particular, to a system and method for providing an integrated optical waveguide device for wavelength multiplexing, wavelength demultiplexing and mode-matching.
2. Related Art
Wavelength division multiplexing (WDM) has been proven as a powerful technology for increasing the capacity of fiber optic communications links as well as in providing all-optical routing and switching of data traveling in such links. Presently, WDM is widely deployed in long haul and metropolitan area networks (MAN). In a WDM fiber optic system, several independent optical signals are transmitted from the transmission end of the system. The optical signals, each having a different optical wavelength, are transmitted through a single optical fiber, either single mode or multi-mode. The single mode fiber is usually used for longer distances, while the multimode fiber is usually used for shorter distances. At the receiving end of the WDM fiber optic system, the different wavelength optical signals are detected and separated in accordance with their wavelength. This allows multiple data channels to be created over one optical fiber. For example, four wavelengths can be used to increase the fiber capacity by a factor of four.
Different versions of WDM are defined in terms of the wavelength, or channel, separations. For narrow channel separation, the term xe2x80x9cdense WDMxe2x80x9d is used, and for wide channel separation, either xe2x80x9ccoarse WDMxe2x80x9d or xe2x80x9cwide WDMxe2x80x9d are used. WDM also provides similar advantages in local area networks (LAN) and access networks. In such networks, WDM offers scalable bandwidth capacity while enhancing the value of the embedded fiber plant, without the need to rewire the premises for future bandwidth upgrades. Furthermore, the parallel nature of the WDM link means that for a given link capacity, lower cost, lower capacity optical/electronic components may be used for each wavelength.
One of the key components in a WDM transmission link is an optical filter. Optical filters serve many functions in a WDM fiber optic system. At the transmission end, optical filters, each with different passband wavelengths, are often used to provide different wavelength channels, allowing the different wavelength channels to be combined together in a multiplexing function. At the receiving end, the optical filter is often used to separate the different wavelength channels in a demultiplexing function. In most cases, the same optical filters are utilized to perform the multiplexing function in the transmission end as well as the demultiplexing function at the receiving end.
As the demand for data speed continues to increase, channel counts in WDM fiber optic systems need to be increased and channel spacing in WDM fiber optic systems need to be narrowed. Presently, the channel counts in conventional WDM fiber optic systems are moving toward several hundred WDM channels and the channel spacing is moving toward 50 GHz to 25 GHz. As a result, it is desirable for WDM fiber optic systems to be made compact, allowing them to scale to large channel count, narrow channel spacing WDM fiber optic systems. Because WDM fiber optic systems should preferably be compact in size, optical filters included in the WDM fiber optical systems also need to be compact. Many different techniques have been developed to realize WDM filters and/or WDM fiber optic systems. Examples of these techniques include arrayed waveguide gratings, Fiber Bragg gratings, and hybrid integration of thin-film filters with waveguides. Disadvantages are, however, associated with each of these techniques.
Arrayed waveguide grating is an attractive technique to manufacture WDM devices with large channel counts. It operates on similar principle as a Mach-Zehnder interferometer. Several copies of the input signal, which are phase shifted by different amounts, are added together in order to realize nxc3x971 wavelength multiplexer or 1xc3x97n wavelength demultiplexer. The input and output waveguides, the multiport couplers and the arrayed waveguides are all fabricated on a single substrate. However, in a system that employs arrayed waveguide gratings, waveguide bends and large arrays of waveguides are required to achieve narrow channel resolution. Due to this requirement, such gratings tend to occupy a large area of the waveguide substrate.
Fiber Bragg gratings are formed by imposing a periodic perturbation of the refractive index in the core of the optical fiber. One method to realize such refractive index changes is to expose the fiber core to ultraviolet light. At a resonant wavelength, i.e., the Bragg wavelength, light is scattered by the periodic index grating from a forward wave into a reflected wave. A circulator is used at the input to extract the resonant wavelength that is propagating in the reverse direction. A circulator is bulky and is required for each Bragg grating. Therefore, the fiber Bragg grating devices cannot be easily scaled to large channel count WDM systems.
Hybrid integrating of thin-film filters with waveguides is typically implemented with Dielectric Thin Film (DTF) Filters. DTF filters are made using well-established techniques for manufacturing optical filters. A desired passband is obtained by cascading several Fabry-Perot (FP) type filters. Each FP filter comprises multi-layers of dielectric films deposited on a substrate that is transparent to the wavelengths of interest. The films are typically deposited by evaporation. The composite structure is then cut into pieces comprising individual filter elements. A wafer upon complete fabrication would yield several thousands of WDM filter pieces. These optical filter pieces are then aligned precisely with the input/output optical fiber and packaged. By choosing the appropriate films and controlling their thickness, the desired center wavelength and the width of the filter passband are engineered.
Several deficiencies, however, are associated with a system having hybrid integration of DTF filters and waveguides. For the present generation of 10 GHz WDM fiber optical systems, 200 or more layers of dielectric films are required in the DTF filter structure. There are already several manufacturing issues with maintaining the uniformity and thickness of the layers in such a complex structure. It is anticipated that more difficulties will be encountered using DTF technology to scale to future 50 GHz and narrower WDM filter optic systems. Furthermore, because of the need to preserve optical, thermal and mechanical properties of the thin-film layer stack, the smallest DTF filter pieces available are approximately 1 mm2. This prevents ultra-small filter structures in the xcexcm range from being created. Moreover, for applications in fiber optic communication, a method must also be devised to couple the light into an array of DTF filters, each having a center wavelength that is identical to one of the WDM channels. This is typically accomplished using reflection in a second dielectric substrate. Precise alignment of the DTF filter with the optical fibers is accomplished using lens couplers or hybrid packaging schemes. Such hybrid packaging schemes require precision alignment steps that are expensive and time-consuming.
In view of the above deficiencies associated with the conventional WDM fiber optic systems, a need exists for a system and method to provide fiber optic systems that are not only compact in size but are also easily scalable to large channel counts and narrow channel spacing. Moreover, the cost of the optical components, such as filters and multiplexers, is dominated by the cost of hybrid packaging the WDM filter with the input and output optical fibers. This is largely due to the manual alignment process and the sub-micron accuracy required for low insertion loss device. Therefore, a need also exists for avoiding expensive and time-consuming optical alignment procedures required with the conventional WDM fiber optic systems. The system and method of present invention is relatively inexpensive to produce and carry out, while being suitable for straightforward and consistent quality manufacturing.